# std::uses_allocator_construction_args

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< cpp‎ | memory

Defined in header `<memory>` |  |
`T` is not a specialization of std::pair |  |
template< class T, class Alloc, class... Args > constexpr auto uses_allocator_construction_args( const Alloc& alloc, Args&&... args ) noexcept; | (1) | (since C++20)
`T` is a specialization of std::pair |  |
template< class T, class Alloc, class Tuple1, class Tuple2 > constexpr auto uses_allocator_construction_args( const Alloc& alloc, std::piecewise_construct_t, Tuple1&& x, Tuple2&& y ) noexcept; | (2) | (since C++20)
template< class T, class Alloc > constexpr auto uses_allocator_construction_args( const Alloc& alloc ) noexcept; | (3) | (since C++20)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc, U&& u, V&& v ) noexcept; | (4) | (since C++20)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc, std::pair<U, V>& pr ) noexcept; | (5) | (since C++23)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc, const std::pair<U, V>& pr ) noexcept; | (6) | (since C++20)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc, std::pair<U, V>&& pr ) noexcept; | (7) | (since C++20)
template< class T, class Alloc, class U, class V > constexpr auto uses_allocator_construction_args( const Alloc& alloc, const std::pair<U, V>&& pr ) noexcept; | (8) | (since C++23)
template< class T, class Alloc, class NonPair > constexpr auto uses_allocator_construction_args( const Alloc& alloc, NonPair&& non_pair ) noexcept; | (9) | (since C++20)

Prepares the argument list needed to create an object of the given type `T` by means of uses-allocator construction.

1) This overload participates in overload resolution only if `T` is not a specialization of std::pair. Returns std::tuple determined as follows:

- If std::uses_allocator_v<T, Alloc> is false and std::is_constructible_v<T, Args...> is true, returns std::forward_as_tuple(std::forward<Args>(args)...).
- Otherwise, if std::uses_allocator_v<T, Alloc> is true and std::is_constructible_v<T, std::allocator_arg_t, const Alloc&, Args...> is true, returns
  std::tuple<std::allocator_arg_t, const Alloc&, Args&&...>(std::allocator_arg, alloc,
  std::forward<Args>(args)...).
- Otherwise, if std::uses_allocator_v<T, Alloc> is true and std::is_constructible_v<T, Args..., const Alloc&> is true, returns std::forward_as_tuple(std::forward<Args>(args)..., alloc).
- Otherwise, the program is ill-formed.

2) This overload participates in overload resolution only if `T` is a specialization of std::pair. For `T` that is std::pair<T1, T2>, equivalent to

    return std::make_tuple(std::piecewise_construct,
        std::apply([&alloc](auto&&... args1)
            {
                return std::uses_allocator_construction_args<T1>(alloc,
                           std::forward<decltype(args1)>(args1)...);
            }, std::forward<Tuple1>(x)
        ),
        std::apply([&alloc](auto&&... args2)
            {
                return std::uses_allocator_construction_args<T2>(alloc,
                           std::forward<decltype(args2)>(args2)...);
            }, std::forward<Tuple2>(y)
        )
    );

3) This overload participates in overload resolution only if `T` is a specialization of std::pair. Equivalent to

    return std::uses_allocator_construction_args<T>(alloc,
        std::piecewise_construct, std::tuple<>{}, std::tuple<>{}
    );

4) This overload participates in overload resolution only if `T` is a specialization of std::pair. Equivalent to

    return std::uses_allocator_construction_args<T>(alloc,
        std::piecewise_construct,
        std::forward_as_tuple(std::forward<U>(u)),
        std::forward_as_tuple(std::forward<V>(v))
    );

5,6) This overload participates in overload resolution only if `T` is a specialization of std::pair. Equivalent to

    return std::uses_allocator_construction_args<T>(alloc,
        std::piecewise_construct,
        std::forward_as_tuple(pr.first),
        std::forward_as_tuple(pr.second)
    );

7,8) This overload participates in overload resolution only if `T` is a specialization of std::pair. Equivalent to

    return std::uses_allocator_construction_args<T>(alloc,
        std::piecewise_construct,
        std::forward_as_tuple(std::get<0>(std::move(pr))),
        std::forward_as_tuple(std::get<1>(std::move(pr)))
    );

9) This overload participates in overload resolution only if `T` is a specialization of std::pair, and given the exposition-only function template

    template<class A, class B>
    void /*deduce-as-pair*/(const std::pair<A, B>&);

, /*deduce-as-pair*/(non_pair) is ill-formed when considered as an unevaluated operand.
Let the exposition-only class `pair-constructor` be defined as

    class /*pair-constructor*/
    {
        const Alloc& alloc_; // exposition only
        NonPair&     u_;     // exposition only

        constexpr reconstruct(const std::remove_cv<T>& p) const // exposition only
        {
            return std::make_obj_using_allocator<std::remove_cv<T>>(alloc_, p);
        }

        constexpr reconstruct(std::remove_cv<T>&& p) const // exposition only
        {
            return std::make_obj_using_allocator<std::remove_cv<T>>(alloc_, std::move(p));
        }

    public:
        constexpr operator std::remove_cv<T>() const
        {
            return reconstruct(std::forward<NonPair>(u_));
        }
    };

This overload is equivalent to return std::make_tuple(pair_construction);, where `pair_construction` is a value of type `pair-constructor` whose `alloc_` and `u_` members are `alloc` and `non_pair` respectively.

### Parameters

alloc | - | the allocator to use
args | - | the arguments to pass to `T`'s constructor
x | - | tuple of arguments to pass to the constructors of `T`'s `first` data member
y | - | tuple of arguments to pass to the constructors of `T`'s `second` data member
u | - | single argument to pass to the constructor of `T`'s `first` data member
v | - | single argument to pass to the constructor of `T`'s `second` data member
pr | - | a pair whose `first` data member will be passed to the constructor of `T`'s `first` data member and `second` data member will be passed to the constructor of `T`'s `second` data member
non_pair | - | single argument to convert to a std::pair for further construction

### Return value

std::tuple of arguments suitable for passing to the constructor of `T`.

### Notes

The overloads (2-9) provide allocator propagation into std::pair, which supports neither leading-allocator nor trailing-allocator calling conventions (unlike, e.g. std::tuple, which uses leading-allocator convention).

When used in uses-allocator construction, the conversion function of `pair-constructor` converts the provided argument to std::pair at first, and then constructs the result from that std::pair by uses-allocator construction.

### Example

| This section is incomplete Reason: no example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3525 | C++20 | no overload could handle non-`pair` types convertible to `pair` | reconstructing overload added

### See also

uses_allocator (C++11) | checks if the specified type supports uses-allocator construction (class template)
make_obj_using_allocator (C++20) | creates an object of the given type by means of uses-allocator construction (function template)
uninitialized_construct_using_allocator (C++20) | creates an object of the given type at specified memory location by means of uses-allocator construction (function template)

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